Guanzhou to Hong Kong

No. Because you will take the train to HK, but the 72 hour policy is only applicable to flight travelers.

Since you'll be driving leaving Guangzhou, and according to this rule, Simon,

"The confirmed seat onward ticket and valid entry permit to a third country or region (if required) are needed for entry."

The 72-hour rule will not apply for you, since you have no onward ticket with a set departure date on it. You will need to have a valid ticket that flies from Guangzhou to HKG, or get a visa. I'm no expert on this: so, others can chime in if I'm wrong.